Preheat oven to 350F. Spray baking dish with nonstick spray.
In a large saucepan, melt butter (1 stick) over low heat to medium heat. Once melted, add in flour and mix until thick.
Pour in milk and half and half. Whisk until combined and smooth. Sprinkle in seasonings, cover, and simmer for 5 minutes. and mix.
Once mixture comes to a boil, start adding cheese in little bits, melting cheese each time. After cheese is combined, stir until smooth.
Pour half the cheese sauce into casserole dish. Cover with cooked pasta. Top with remaining sauce.
In a small bowl, combine 3 tablespoons of butter and Ritz crackers. Layer over the top of mac and cheese. Bake for 30 minutes. Allow to cool slightly before serving.